A mass spectrometer (MS) is a device that filters gaseous ions according to their mass-to-charge (m/z) ratio and measures the relative abundance of each ionic species. Mass spectrometry is particularly attractive for in-situ analysis, due to its inherent speed, excellent sensitivity, molecular selectivity, and capability for continuous real-time measurements. A typical mass spectrometer comprises an ion source, wherein the ions are generated; a mass filter, wherein the ions are separated in space or in time; an ion detector, wherein the filtered ions are collected and their relative ion abundance measured; a vacuum system; and means to power the spectrometer. Depending on the type of sample and the method of introducing the sample into the mass spectrometer, ions can be generated in the ion source by electron impact ionization, photoionization, thermal ionization, chemical ionization, desorption ionization, spray ionization, or other processes. Mass spectrometers are generally classified according to the method on which mass filtering is accomplished using electric and/or magnetic fields. Mass filter types include magnetic-sector, time-of-flight, linear quadrupole, ion cyclotron resonance, and ion traps. Detection of ions is typically accomplished by a single-point ion collector, such as a Faraday cup or electronic multiplier, or a multipoint collector, such as an array or microchannel plate collector, whereby all of the ions arrive at the collector simultaneously.
Mass spectrometer performance is generally given in terms of mass range, resolution (i.e., resolving power), and sensitivity of the instrument. Mass range is the lowest and highest masses that can be measured. A large mass range is desired for the analysis of high molecular weight organic and biological analytes. Resolution measures the ability of the instrument to separate and identify ions of slightly different masses. Typically, the resolution for singly charged ions is given by
  R  =      m          Δ      ⁢                          ⁢      m      where m is the mass of an ion peak in atomic mass units and Δm is the width of the peak at some peak height level (e.g., half peak height). In many cases, the minimum resolution required is such that a molecular ion can be resolved from an adjacent peak having a unit mass difference. According to this requirement, the resolution R should be at least 100 for a chemical species having a nominal mass of 100. High-resolution instruments, required for organic mass spectrometry, can detect peaks separated by fractions of a mass unit. Sensitivity is a measure of the instrument's response to ions of an arbitrary m/z ratio for a particular sample. Sensitivity is typically a function of the efficiency of the ion source and ion detector, as well as the analyzer method used. The sensitivity limit, or detection limit, is the minimum amount of a sample that can be detected under a given set of experimental conditions and distinguished from the instrument noise level and background. Resolution and sensitivity are approximately inversely related to each other. Other important characteristics of a spectrometer instrument include overall size, operating pressure, voltage, and power consumption.
Mass spectrometers can be used for chemical sensing. A pulse of sample gas can be injected directly into the ion source of the mass spectrometer. However, analyzing mixtures may be difficult when the mass spectrometer is used alone, since the resulting mass spectrum would be a complex summation of the spectra of the individual components. Therefore, analytical techniques combining the separation methods of gas chromatography and mass spectrometry are often used for chemical sensing. A gas chromatograph (GC) separates volatile mixtures into their component chemical species, which are eluted from a long capillary. The eluents can then be transferred into a mass spectrometer to obtain a mass spectrum of each of the separated components, from which the molecular structure of the individual component species can be inferred. The GC/MS is therefore capable of separating highly complex mixtures, identifying the components, and quantifying their amounts. Alternatively, tandem (MS/MS) or multistage (MSn) mass spectrometers can be combined, wherein one of the mass spectrometers is used to isolate individual ions according to their m/z ratio, and the other is used to examine the fragmentation products of the individual ions. Thus, multiple stages of mass analysis can be obtained in a single analyzer.
Recently, there has been a growing interest in miniature mass spectrometers that enable reduced size, power requirements, vacuum system demands, cost, and complexity. In particular, a microfabricated mass spectrometer combined with a microfabricated GC would enable a very attractive portable, handheld microanalytical system. Such a “chemical laboratory on a chip” would enable the rapid and sensitive detection of particular chemicals, including pollutants, high explosives, and chemical and biological warfare agents in the field.
Because magnetic separators do not scale well into the microstructure size range, most microfabricated mass filters have performed separation using time-varying electric fields (i.e., Paul-type traps). The 3D quadrupole ion trap (QIT) consists of two hyperbolic endcap electrodes with their foci facing each other and a hyperbolic ring electrode halfway between the two endcap electrodes. Ions are trapped in the space between these three electrodes by RF and DC electric fields. The QIT is well suited to miniaturization due to pressure intolerance and low voltage and power requirements. Cylindrical ion traps (CIT), comprising flat endcap electrodes and a cylindrical ring electrode, have much simpler geometries than the QIT. Massive arrays of such CITs have been fabricated on a micro-scale. See U.S. Pat. Nos. 6,870,158 and 7,154,088. However, such CITs are difficult to fabricate on the micro-scale, require high RF driving powers, and require unconventional ion detection schemes due to their relatively closed geometry.
The linear Paul ion trap is a two-dimensional version of the 3D QIT. A quadruple mass spectrometers based on the conventional linear Paul trap filters ions by passing them through tuned radiofrequency (RF) and direct current (DC) electrical fields defined by four symmetrically-spaced parallel rods within which a 2D quadrupole field is established. The QMS permits only those ions with a stable trajectory, determined by their m/z ratio, to travel along the entire length of the central axis of the rod assembly without being deflected out of the intra-rod space. Ions with different m/z ratios can be scanned through the QMS by continuously varying the field between the quadrupole rods. Therefore, the QMS is a variable bandpass filtering ion optic.
The QMS is relatively pressure tolerant and can operate effectively at relatively high pressures (e.g., 10−4 Torr). Therefore, they are amenable to miniaturization due to the avoidance of bulky vacuum pumping systems. Miniature linear quadrupoles require lower drive voltages and higher RF drive frequencies to filter heavier ions and maintain resolution as the electrode dimensions decrease. Further, the linear ion trap has much greater efficiency for trapping externally generated ions and much greater ion trapping capacity, due to reduced space charge effects, than 3D QITs of the same volume. However, miniaturized linear ion traps require precise fabrication to maintain acceptable performance. See Z. Ouyang et al., Eur. J. Mass Spectrom. 13, 13 (2007).
Recently, there has been interest in microfabricated versions of the linear ion trap for quantum computing (QC). To increase the speed of QC operations, there has been an effort to reduce the size of trap dimensions. This has lead to the development of a microfabricated surface-electrode ion trap analog of the conventional four-rod Paul ion trap. With the surface-electrode ion trap, all of the trap electrodes reside on a single surface, with alternating flat RF and DC control electrodes, and the ions are trapped above the surface plane. Ions are confined transversely to the null ion trap axis of the RF quadrupole electric field and axial confinement and ion transport operations are controlled by the DC electric field. For QC applications, the control electrodes can be segmented along their length and appropriate static potentials can be applied to the different segments to enable longitudinal confinement of ions in a single trapping location and transportation of the ions between trapping locations. However, ion trap depths of the surface-electrode trap are very shallow, not much above the mean kinetic energy of the ions. Therefore, ion injection and trapping with a thermal ion source can be problematic. However, shallow pseudopotential depths are tolerable for quantum computing applications which use laser cooling to initialize the internal state of the ions. See J. Chiaverini et al., Quantum Inf. Comput. 5, 419 (2005); S. Seidelin et al., Phys. Rev. Lett. 96, 253003 (2006); and C. E. Pearson et al., Phys. Rev. A 73, 032307 (2006).
Many of the requirements for micro-scale ion traps for mass spectrometry differ from those for quantum computing. The basic requirements for a useful micro-scale ion trap for mass spectrometry are: an ionization method that enables ionizing many different atomic and molecular species such that nearly all component species in the sample can be ionized for analysis; an ability to trap many target species to enable species identification and the determination of relative concentrations of all of the components in the sample; and an ability to trap a large quantity of ions to enable direct detection of the ions with adequate sensitivity for analysis.
The present invention is directed to a microfabricated linear Paul-Straubel ion trap array that satisfies these requirements for mass spectrometry in a small package.